1. Field of the Invention
The present invention relates to an exhaust purification device for an internal combustion engine having a NOx occlusion reduction type catalyst in an exhaust pipe.
2. Description of Related Art
As a conventional technology, there is an exhaust purification device of an internal combustion engine that has a catalyst in an exhaust system for promoting a reduction reaction of NOx and that grasps a combustion state of the engine from a fuel injection quantity and engine rotation speed. The exhaust purification device selects a suitable control method from multiple methods (rich combustion, a post-injection, low-temperature combustion based on EGR control, fuel addition to the exhaust system through a fuel addition valve) according to the combustion state. Thus, the technology performs an efficient reduction reaction of the NOx (for example, as described in Patent document 1: JP-A-2003-120392).
Conventionally, it is known that in order to achieve both of a NOx purification rate and suppression of a discharge quantity of particulate matters (i.e., PM), it is effective to provide multiple methods and to selectively use a method suitable for a situation out of the multiple methods. The multiple methods include a method of reducing the NOx by a method of performing combustion at a stoichiometric air fuel ratio or under by increasing a main injection quantity or by a method of performing a subsidiary injection at such a short interval immediately after the main injection that the subsidiary injection generates torque (hereinafter, referred to as rich combustion) in a low rotation speed and low load area and include a method of reducing the NOx by a method of exhaust gas fuel addition for additionally providing a fuel addition valve at a position upstream of the catalyst in the exhaust system and for injecting the fuel (as a reducing agent) directly to the catalyst or by a method of performing a post-injection for performing a subsidiary injection at a long interval after the fuel injection in a high rotation speed and high load area. However, there are some problems that should be taken into consideration when performing such the selective use. That is, for example, when the rich combustion is performed in the high rotation speed and high load area such as high-speed travel on an expressway, an ignition delay is short and the fuel is combusted before sufficiently mixed with an air. As a result, a large quantity of smoke is generated and deposition of the particulate matters in a diesel particulate filter (hereinafter, referred to as a DPF) is promoted. In this case, regeneration of the DPF for combusting and removing the particulate matters deposited in the DPF is performed frequently, so the fuel is used for combusting and removing the particulate matters and a fuel consumption is deteriorated. Therefore, the rich combustion should be preferably avoided in the state of the high rotation speed and high load area.
When there occurs a condition that makes it difficult for the fuel to combust (for example, when the engine is operated at a high altitude or when low-cetane fuel is used), the fuel becomes less ignitable and therefore there is a possibility that an appropriate driving force cannot be obtained due to a misfire (i.e., torque shock can occur) if the rich combustion is performed on the same condition as a normal period. Therefore, when the operation at the high altitude is performed or when the low-cetane fuel is used, the rich combustion in the state of the low rotation speed and low load should be preferably avoided.
The cetane number is an index for measuring ignitability of the fuel. Generally, even if certain fuel is called as the low-cetane fuel, the cetane number of the fuel varies, e.g., depending on a gas station where the fuel is obtained. In order to directly measure the cetane number of the fuel, components of the fuel have to be analyzed. However, such the measurement is difficult in the present stage, so another index is necessary. At the high altitude, the atmospheric pressure is low and the ignition of the fuel becomes difficult. Therefore, at the high altitude, a phenomenon similar to the case of using the low-cetane fuel occurs.
As described above, a large quantity of the smoke is generated if the rich combustion is performed in the state of the high rotation speed and the high load. Because of the large quantity of the smoke, a large quantity of the particulate matters are deposited inside the DPF, necessitating frequent regeneration of the DPF. As a result, the deterioration of the fuel consumption is accelerated. In order to appropriately deal with the deterioration of the fuel consumption, it is indispensable to measure the discharge quantity of the smoke. However, it is difficult to directly measure the discharge quantity of the smoke, so a certain index is necessary. Therefore, the quantity of the particulate matters deposited in a filter section provided on an exhaust side of the engine may be used as an index of the discharge quantity of the smoke. It is because when a large quantity of the particulate matters are deposited in the filter section, it means that a large quantity of the smoke has been discharged.